Reactions of Frustrated Lewis Pairs with Chloro‐Diazirines: Cleavage of N=N Double Bonds

Abstract The reactions of FLPs with diazomethanes leads to the rapid loss of N2. In contrast, in this work, we reported reactions of phosphine/borane FLPs with chlorodiazirines which led to the reduction of the N=N double bond, affording linked phosphinimide/amidoborate zwitterions of the general form R3PNC(Ar)NR′BX(C6F5)2. A detailed DFT mechanistic study showed that these reactions proceed via FLP addition to the N=N bond, followed by subsequent group transfer reactions to nitrogen and capture of the halide anion.


General information for synthesis
Experiments were carried under inert conditions using standard Schlenk techniques or a glove box as appropriate. Dichloromethane (DCM, CH2Cl2) and n-hexanes (C6H14) were dispensed from an MBRAUN Solvent Purification System, deoxygenated by bubbling Ar for 20 min, and stored over 3 Å molecular sieves prior to use whilst 1,2-dichloroethane (1,2-DCE, 1,2-C2H4Cl2), was stirred over CaH2 at room temperature under Ar overnight prior to distillation under reduced pressure and stored under 3 Å molecular sieves before use.
Chloroform-d (CDCl3) and dichloromethane-d2 (CD2Cl2) solvents were used as received without any purification and those were stored over 4 Å molecular sieves prior to use. Vials and stir bar for reactions were oven-dried overnight before experiments. 1 H (500 MHz), 19 F (471 MHz), 19 F{ 1 H} (471 MHz), 31 P{ 1 H} (202 MHz), and 13 C{ 1 H} (126 MHz) NMR spectra were run at 298 K on Bruker 500 spectrometers. The chemical shifts (δ, ppm) for 1 H and 13 C{ 1 H} NMR spectra are given relative to solvent signals whereas an external reference standards used for 31 P{ 1 H} (85% H3PO4), 19 F (CFCl3) and 19 F{ 1 H} (CFCl3) NMR spectra. These NMR data are written as: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, br = broad), coupling constants (Hz) and integration. The single-crystal X-ray data were collected on a Bruker D8 QUEST diffractometer using Cu (60W, Diamond, μKα = 12.894 mm -1 ) micro-focus X-ray sources at 150 K. The structure was solved and refined using Full-matrix least-squares based on F 2 with a suite of programs SHELXS and SHELXL 1 compiled in OLEX2. 2 The reagents B(C6F5)3 3 and diazirines 4 were prepared by following literature method or a slight variations thereof. All other reagents were purchased commercially and used as received.

S6
Synthesis of 2 2 (81 mg, 92%) was prepared by following the protocol for 1 whilst the reaction vial was heated at 40°C for 24 h. X-ray quality crystals were grown with a mixture of solvent of DCM:n-hexane (1:5) and stored at -30°C for two days.

Synthesis of 3
3 (78 mg, 84%)was prepared by following the protocol for 1 whilst the reaction vial was heated at 40°C for 24 h. X-ray quality crystals were grown with a mixture of solvent of DCM:n-hexane (1:5) and stored at -30°C for two days. 3: 1

Synthesis of 6
S9 6 (64 mg, 77%) was prepared by following the protocol for 1 whilst the reaction vial was heated at 40°C for 24 h

Synthesis of 9
9 (60 mg, 69%)was prepared by following the protocol for 1 whilst the reaction vial was heated at 40°C for 24 h. X-ray quality crystals were grown with a mixture of solvent of DCM:n-hexane (1:5) and stored at -30°C for two days. 9: 1

Synthesis of 10
Into a 4 mL open top PTFE vial equipped with a stir bar, compound 1 (52.4 mg, 0.05 mmol, 1.0 equiv.) was dissolved in DCM (1.0 mL). After addition of Et3SiH (9.6 μL, 0.06 mmol, 1.1 equiv.), the reaction mixture was allowed to stir at 40 ℃ for 24 h. After removal of all volatiles, the residue was washed with n-hexane (3 x 1 mL). Further, the residue was dried in affording compound 10 (44 mg, 87%). X-ray quality crystals were grown with a mixture of solvent of DCM:n-hexane (1:5) and stored at -30°C for two days. Due to limited solubility partial characterization data included here. 10: 1
After addition of a solution of 3-(4-bromophenyl)-3-chloro-diazirine (23mg, 0.1 mmol, 1.0 equiv.) in DCM (0.5 mL), the reaction mixture was allowed to left at RT for 24 h. The mixture was analyzed by NMR spectroscopy. Figure S59. Image for control reactions after 24 h at RT.

Computational Details
The quantum chemical DFT calculations have been performed with the TURBOMOLE 7.4 suite of programs [1] The initial structures generated according to their Lewis structures are checked with the CREST method using the xTB program for low-lying conformers as input. [2] The structures are fully optimized at the TPSS-D3/def2-TZVP + COSMO level of theory, which combines the TPSS meta-GGA density functional [3] with the BJ-damped DFT-D3 dispersion correction [4] and the def2-TZVP basis set, [5] using the Conductor-like Screening Model (COSMO) continuum solvation model [6] for CH2Cl2 solvent (dielectric constant  = 8.93 and solvent diameter Rsolv = 2.94 Å). The density-fitting RI-J approach [5a, 7] is used to accelerate the geometry optimization and numerical harmonic frequency calculations [8] in solution. The optimized structures are characterized by frequency analysis to identify the nature of located stationary points (no imaginary frequency for true minima and only one imaginary frequency for transition state) and to provide thermal corrections (at 298.15 K and 1 atm) according to the modified ideal gas  rigid rotor  harmonic oscillator model. [9] This choice of dispersion-corrected meta-GGA functional makes the efficient exploration of all potential reaction paths possible.
The final solvation free energies in CH2Cl2 solution are computed with the COSMO-RS solvation model [10] (parameter file: BP_TZVP_C30_1601.ctd) using the COSMOtherm program package [[11] on the above TPSS-D3 optimized structures, and corrected by +1.89 kcal·mol  1 to account for higher reference solute concentration of 1 mol·L −1 usually used in solution. To check the effects of the chosen DFT functional on the reaction energies and barriers, single-point calculations at the meta-GGA TPSS-D3 [3] and hybrid-meta-GGA PW6B95-D3 [12] levels are performed using a larger def2-QZVP basis set. [5b, 13] The final reaction Gibbs free energies (G) are determined from the electronic single-point energies plus TPSS-D3 thermal corrections and COSMO-RS solvation free energies. In our discussion, higher-level PW6B95-D3 Gibbs free energies (in kcal/mol, at 298.15 K and 1 mol/L concentration) will be used in our discussion unless specified otherwise. The applied S55 DFT methods in combination with the large AO basis set provide usually accurate electronic energies leading to errors for chemical energies (including barriers) on the order of typically 1-2 kcal/mol. This has been tested thoroughly for the huge data base GMTKN55 [14] which is the common standard in the field of DFT benchmarking. To help experimental NMR assignment, nuclear magnetic shielding constants are computed using the GIAO (Gauge Including Atomic Orbital) method [15] at the TPSS/def2-QZVP level; final 31 P, 11  The diazirine ArCClN2 (Ar = p-C6H4Br) may directly abstract a (C6F5)2BH unit from the stable Lewis adduct (C6F5)2BH  P(o-tol)3, which is 5.7 kcal/mol endergonic over a sizable barrier of 22.4 kcal/mol (via TS5) to form the Lewis adduct E along with released base P(otol)3; further P…N addition between E and P(o-tol)3 is still 11.1 kcal/mol exergonic over a low barrier of 10.7 kcal/mol (via TS6), making the formation of F as the FLP adduct of The diazirine ArCClN2 (Ar = p-C6H4Br) may directly abstract a B(C6F5)3 unit from the stable Lewis adduct (C6F5)3BPHPh2, which is 14.7 kcal/mol endergonic over a moderate barrier of 18.9 kcal/mol (via TS8) to form the Lewis adduct A along with released base HPPh2; further P…N addition between A and HPPh2 is still 17.7 kcal/mol exergonic over a very low barrier of 4.3 kcal/mol (via TS9), making the formation of I as the FLP adduct of ArCClN2 3.0 kcal/mol exergonic from the stable Lewis adduct (C6F5)3BPHPh2. Table S1. TPSS-D3/def2-TZVP + COSMO computed imaginary frequency (ImF), zero-point energies (ZPE), gas-phase enthalpic (Hc) and Gibbs free-energy (Gc) corrections; the COSMO-RS computed solvation enthalpic (Hsol) and Gibbs free-energy (Gsol) corrections in CH2Cl2 solution; TPSS-D3/def2-QZVP and PW6B95-D3/def2-QZVP single-point energies (TPSS-D3 and PW6B95-D3); the total PW6B95-D3 free energies GP; the relative electronic energies (ET and EP) and Gibbs energies (GT and GP) at the TPSS-D3 and PW6B95-D3 levels. (